The invention relates to a method for operating an internal combustion engine operated with a self-ignitable fuel, with an approximately homogeneous fuel-air mixture being produced in the combustion chamber in the self-ignition ranges, a high compression ratio suitable for a self-ignition of the fuel being provided and the combustion occurring in the self-ignition ranges predominantly by self-ignition, as well as an internal combustion engine to perform the method.
From the publication xe2x80x9cHomogeneous Charge Compression Ignition (HCCI) of Diesel Fuelxe2x80x9d, Allen W. Gray et al., SAE 971676, it is known that during the combustion of a lean fuel-air mixture ignited by self-ignition extremely low emission values for NOx and exhaust particulates are achieved as a result of the homogeneous distribution of concentration and temperature. This method is known in the English-speaking countries as the HCCI method (Homogeneous Charge Compression Ignition) . It is further known that diesel fuel causes difficulties in this combustion method because the time of ignition can be fixed due to its high ignition performance only in the desired manner at a low compression ratio and low mean effective pressure shortly before the upper dead center. The required low compression ratio of approx. 10 leads to considerable disadvantages concerning the specific fuel consumption and the achievable output yield as compared with conventional diesel methods which to date have prevented the wide-scale use of this actually very low-emission combustion method. A further difficulty relating to the diesel fuel is the position of the boiling range of between 170xc2x0 C. and 360xc2x0 C. which is obstructive to the evaporation and thus to the homogenization of the cylinder charge and which can lead to high emissions of NOx, exhaust particulates and non-burned hydrocarbons and the likelihood of enrichment of diesel fuel in the lubricating oil.
Due to its very low ignition performance and the lower boiling range of between approx. 30 and 190xc2x0 C., gasoline has clear advantages for the HCCI method. The compression ratio can be raised to values of approx. 15 to 17, as in the diesel engine. However, the achievable mean effective pressure is limited in a disadvantageous manner in this case too to the part-load range, as is disclosed in the publication xe2x80x9cAn Experimental Study on Premixed-Charge Compression Ignition Gasoline Enginexe2x80x9d, Taro Aoyama et al., SAE 960081.
Furthermore, from DE 36 32 579 C2 a spark-ignited, air-compressing internal combustion engine is known which operates like a diesel engine at a high compression ratio, in which a charge stratification is produced in order to ensure the ignition of the air-fuel mixture. This stratified charge combustion method is also known as the SCSI method (Stratified Charge Spark Ignition). The principle of the stratified charge ensures that the mixture composition remains long enough within the ignition limits in the zone of the ignition source during the spark discharge in order to produce a flame which is sufficiently large enough for the continued combustion of the cylinder charge. Such a stratified charge combustion method with spark ignition is not as advantageous with respect to emissions as the HCCI method, but can be operated with much higher mean pressures and is much better than the diesel engine concerning exhaust particulate emissions.
A method is known from DE 28 51 504 A1 for operating an internal combustion engine with a variable compression ratio, with the internal combustion engine being spark-ignited with one type of fuel and being operated with self-ignition. In this internal combustion engine the compression ratio to perform the self-ignition is increased in the part-load range and reduced in the full-load range for spark-ignition operation. Fuel for diesel operation is supplied by way of an injection nozzle opening into the main combustion chamber or a pre-combustion chamber, thus obtaining an inhomogeneous fuel-air mixture in the combustion chamber. As a result, there is no HCCI operation. The supply of the fuel for the spark-ignition operation is performed in one embodiment via a carburetor and in another embodiment via a further injection nozzle opening into a side combustion chamber. The combustion is initiated in the spark-ignition operation by way of a spark plug opening into the side combustion chamber. In the case of spark ignition with fuel injection and air intake in full-load operation, there is a charge concentration in the side combustion chamber relative to the main combustion chamber. The known method allows a stable operating behavior and a favorable efficiency. The extremely low emission values as known from the HCCI method cannot be achieved, however.
U.S. Pat. No. 4,126,106 A also describes an internal combustion engine which can be operated both according to the Otto as well as the diesel method. A stratified charge is produced during the starting phase and the part-load operation under direct injection of fuel into the combustion chamber and a combustion by spark ignition of said stratified charge is initiated. Under full load, however the fuel is injected directly into the combustion chamber against the hot walls of the combustion chambers, with the fuel evaporating and the ignition being performed by compression according to the diesel method. The engine is operated at a compression ratio of less than 16:1. At a higher engine load the time between the start of the injection and the time of the self-ignition is insufficient for a favorable preparation of the mixture, which is why a deterioration of the combustion sequence and the emissions occurs. Furthermore, from U.S. Pat. No. 3,125,079 A, a multi-purpose internal combustion engine is known which can be operated both with self-ignition as well as spark ignition with a fixed compression ratio of 15:1. The fuel is injected directly radially into the combustion chamber via a multi-jet injection nozzle. This is not suitable for producing a high degree of charge stratification.
In order to improve the exhaust gas quality in an internal combustion engine for spark- and self-ignitable fuel while simultaneously providing a high efficiency, it is proposed in AT 003 135 U1 to produce an at least approximately homogeneous fuel-air mixture in the combustion chamber in the self-ignition range. This method comprises the advantages of the HCCI method and avoids its disadvantages under high engine load by changing over to spark ignition. The method thus combines the advantages of the HCCI method with those of the SCSI method. However, the start of the combustion of the homogeneous fuel-air mixture can only be controlled with difficulty, because in order to attain homogenization, the formation of the mixture occurs in the suction pipe or very far before the start of the combustion. Accordingly, there is no direct possibility to control the start of the combustion by the start of the injection (as in the diesel engine) or by the ignition spark (as in the Otto engine).
An HCCI engine would require a complex control system for considering or setting a large number of operational parameters for the purpose of controlling the start of the ignition. This would lead to high requirements in respect of control.
It is the object of the present invention to avoid such disadvantages and to control in the simplest possible way the start of the combustion in the self-ignition range in an internal combustion engine operated with a self-ignitable homogeneous fuel-air mixture.
This occurs in accordance with the invention in such a way that in the self-ignition range the pressure and/or temperature of the homogeneous fuel-air mixture are set in such a way, preferably depending on the charge composition and/or charge temperature, that the self-ignition would be achieved only after the actually desired start of combustion in the zone of the upper dead center and the combustion of the homogeneous fuel-air mixture is initiated only by an external energy impulse. The charge is therefore led up to the self-ignition capability only up to a certain distance. The initiation of the combustion is performed by an external energy impulse. This energy impulse is used to raise the energy level of the charge up to self-ignition capability, whereupon the entire cylinder charge reaches the self-ignition capability following a short further delay in ignition and burns out.
The external energy impulse can be an electric ignition spark. It can also be provided, however, that the combustion of the homogeneous fuel-air mixture is initiated in the self-ignition range by the self-ignition of a pilot fuel quantity injected directly before the start of the combustion.
The method can be preferably used for operating an internal combustion engine operated both with spark- as well as self-ignitable fuel, especially gasoline, with the engine operating range being associated with self-ignition and spark-ignition ranges and the combustion in the spark-ignition range being initiated by the spark ignition of the fuel-air mixture and with the part-load range being assigned to the self-ignition range, the full-load range and/or engine operating ranges with high engine load as well as the cold start to the spark-ignition range. In this way the advantages of the HCCI method can be utilized and, by changing over to spark ignition, its disadvantages under high engine load can be avoided. In addition to the self-ignition ranges, a homogeneous fuel-air mixture can be provided in the combustion chamber in at least one spark-ignition range. The homogeneous fuel-air mixture can be stoichiometric or lean. Particularly low fuel consumptions and low emissions can be achieved, however, when a stratified charge is produced in the combustion chamber in the spark-ignition ranges.
At a higher engine load, the time between the start of the injection and the time of the self-ignition would not be sufficient in the HCCI method for a favorable preparation of the mixture, which is why a deterioration of the combustion sequence and the emissions would occur in the case of a self-ignition. Since a spark ignition occurs in the full-load range and/or in engine operating ranges with high engine loads, such disadvantages are avoided. Emissions can thus be improved considerably both in the part-load as well as the full-load range.
Although the SCSI method is not as favorable as the HCCI method with respect to emissions, it still offers the two following advantages as compared with the DI method (direct injection) of a conventional diesel engine with respect to the formation of emissions. On the one hand, the lower tendency of gasoline to form exhaust particulates as compared with diesel fuel is mainly due to the smaller size of the molecules and the concomitant vapor pressure which benefits the formation of the mixture. On the other hand, the method feature of spark ignition offers the additional element of freedom as compared with the diesel engine to be able to strongly increase the time between the start of ignition and the ignition by choosing the distance between start of ignition and time of ignition, thus clearly extending the time available for preparing the mixture, with the stratified charge principle preventing that the quantity of pre-mixed charge which is formed before the ignition and thus the initial speed of pressure rise would become to high.
In engine operating ranges with a homogeneous fuel-air mixture in the combustion chamber, the homogenization of the fuel-air mixture can preferably be performed by external mixture formation, preferably by injection of the fuel into the suction pipe. In engine operating ranges with a stratified charge in the combustion chamber, however, the fuel is always injected directly into the combustion chamber.
The homogenization of the fuel-air mixture in the self-ignition range can also occur by direct injection of the fuel into the combustion chamber. For this purpose the injection device can be designed with changeable injection characteristics. Changeable injection characteristics can be achieved with stroke-variable injection nozzles or with double-needle injection nozzles. A homogenization of the fuel-air mixture can also be realized by different injection pressures of the direct-injecting injection device. In order to allow the defined control of the mixture formation also via the jet quality, an air-supported injection device may be advantageous.
The changeover between self-ignition and spark ignition is performed depending on the mean effective pressure of the work process at a predetermined limit value of the mean effective pressure. The self-ignition range lies below said limit value and the spark ignition range above said spark ignition range. The limit value is defined in a range of the mean effective pressure in which the induced mean pressure is between approx. 4 to 9 bars, preferably between approx. 6 to 9 bars, and especially preferably between approx. 7 to 8.5 bars, with the speed of pressure rise of the cylinder pressure preferably being smaller than or equal to 5 bars per degree of crank angle. The method in accordance with the invention can be used to define the time of the start of combustion of the homogeneous fuel-air mixture in the self-ignition range to such a precise extent as previously was only possible in non-homogeneous self-igniting charges by the injection time or, in the case of spark-igniting charges, by the ignition point.